eRocks

1.3.1 Bond Work Index (BWI)
From work by Bond [1952] a relationship has been derived to determine the Bond Work Index of an ore sample in a standard way. The formula states:

Where
W = work required
Wi = work index
P = 80% passing size of product in microns
F = 80% passing size of feed in microns.

The test is carried out in a standard Bond Mill, in batch-wise, successive grinding cycles. When a circulatory load of 250% is achieved the power required to grind an ore from an infinite size to the desired product size can be calculated. This value in KWh/tonne can then be used to estimate the most suitable mill for the required throughput and ore grindability. Diester [1987] gives a full methodology for the test. The full test from start to finish can require in excess of 30 Kg of sample to ensure representivity. To perform a full Bond test can require and as much as two full operator days. The volume of material needed to perform the test means that exploration drill core samples are not suitable and as a result of this potentially large variations in grindability are not accounted for over the lifecycle of the mine. This factor could be costly, because additional plant may have to be augmented to the original circuit. The method is considered to have a number of flaws which can lead to inaccuracies [Prasher, 1987].

1.3.2 Relative work Index
Due to the complex test procedure and time consuming nature of the full BWI test attempts have been made to simplify and shorten the procedure. Berry and Bruce [1966] devised a method that compares the grinding characteristics of an ore of known BWI and an unknown ore. The method works on the principle that for an identical mill under the same conditions the power draw will be the same. As the BWI of the reference ore is known a simultaneous equation can be used to calculate the unknown ores relative BWI from any shift in the product size distribution produced.

Magdalinovic [1987] proposed a method of determining the BWI that only requires two grinding cycles, as opposed to a typical 7-10 in the standard test. It was shown that this method does not differ by more than 7% when compared to the full BWI test.

1.3.3 Pilot Scale Testing
Pilot scale testing is usually performed prior to any large-scale capital investment to ensure that the grinding characteristics of the rock are suited to the chosen equipment. The process involves feeding enough sample through a scaled down mill so a suitable power requirement can ascertained. From this power requirement final design calculations can be performed. The quantities involved can be in the region of 1-2 tonnes per hour, ensuring that recorded figures are as representative as possible.

1.3.4 SAG Power Index
Work reported by Minnovex Technologies Inc has investigated the relationship between SAG/AG mill and the power consumption. The test method requires as little as 2 kg of ore sample. The small amounts needed mean that exploration drill cores could be used. In conjunction with other mechanical tests (point load) the grindability of the ore can be determined, and when used in conjunction with the BWI, the total energy requirement of the comminution process can be defined.

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